The present invention relates to a dichromatic polarizing element having a high degree of polarization using a water-soluble organic dichromatic dye selected from the group consisting of a bisazo compound, the tautomer thereof, and the salts thereof.
Furthermore, the invention relates to a method of producing a long dichromatic polarizing element capable of obtaining dichromatic polarizing element chips with a good yield.
A polarizer used for a liquid crystal display device is hitherto been obtained by holding between protective films such as triacetyl cellulose a polarizing element obtained by a method of dissolving or adsorbing a dichromatic molecule such as iodine or a dye, etc., in or on an orientation controller made of a macromolecular substance such as polyvinyl alcohol, etc., and stretching the film in one direction to arrange the dichromatic molecule, or a method of adsorbing the above-described dichromatic molecule to a film stretched to a uniaxial direction.
However, by the above-described method, the orientation controller for orienting the dichromatic molecule must be stretched, and thus, there is a restriction that only a polarizer oriented in only one direction can be produced.
On the other hand, recently, as described in JP-A-7-261024 (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d), a polarizer, which requires no stretching step and has an optional polarization axis, formed by forming a molecular layer showing a dichromatic property on a layer having a photoactive molecule formed on a substrate is reported, but there are problems that because the dichromatic molecule is oriented to a specific direction by the irradiation of light, a very long time is required and long polarizer cannot be produced continuously and also dichromatic molecule orientation is poor in uniformity. Furthermore, the above-described polarizer is low in the polarization degree as compared with the conventional polarizers, and thus cannot be practically used by any possibility.
Also, a method of, after applying a rubbing treatment of rubbing with a cloth, a paper, etc., the surface of a glass or a high molecular film in one direction, adsorbing a dichromatic molecule on the surface is proposed (see, J. F. Dreyer, xe2x80x9cJournal of Physics and Colloid Chemistryxe2x80x9d, Page 52, 808 (1948)). However, in the method, a continuous treatment of a long sheet of polarizer is not described, and there are problems that under a high temperature and high humidity, the high molecular film oriented by the rubbing treatment is relaxed, the orientation of the dichromatic molecule is disturbed, the polarization degree is lowered.
Furthermore, in JP-A-3-54506 and JP-A-3-58004, it is proposed to prepare a polarizer by rubbing a triacetyl cellulose film or a polyethylene terephthalate film and adding a dye, but in the method, the polarization degree is low.
On the other hand, in U.S. Pat. Nos. 2,400,877 and 2,544,659, a method of producing a light polarizing element by coating a solution of a dichromatic material on the surface of a substrate, simultaneously with evaporating the solvent from the surface of the substrate, orienting the molecule of the dichromatic material as a nematic phase, and moderately solidifying the molecule in the oriented state is proposed. However, these known light polarizing elements have a low polarization degree, are insufficient in the heat resistance, and cannot be endure for the practical use.
Recently, about a liquid crystal display device, thinning and light-weighting have been progressed, and all members in relation to the display device has been small-sized, thinned, and light-weighted. However, various attempts have been tried as described above, but the polarizer having the desired performance has not yet been obtained at present.
As the conventional orientation controllers, in the method of preparing a polarizer using a polyvinyl alcohol film, etc., as described above, the stretched polyvinyl alcohol is liable to be splitted to the stretched direction, whereby there is a problem that a protective layer must be formed to both side of the polarizer, which increases the thickness of whole the liquid crystal display device. Furthermore, in the conventional method of preparing a polarizer by stretching a polyvinyl alcohol, because the stretched polyvinyl alcohol film is shrunk by heat and humidity, there is a problem that an adhesive existing between the glass surface of a liquid crystal cell and the polarizer is released.
Moreover, for preparing a long polarizer, the polyvinyl alcohol film is oriented to only the lengthwise direction or the width direction, whereby the polarization axis inevitably becomes parallel with or vertical to the lengthwise direction and thus, there is a problem that in the case of attaching the polarizer to a liquid crystal display device, the polarizer is stamped to a rectangle so that the polarization axis becomes the direction of 45 degree, which greatly reduces the yield, and thus it has been desired to solve these problems.
An object of the invention is to provide a dichromatic polarizing element, which can be simply produced, has a high polarization degree, and can cope with small-sizing, thinning, and light-weighting of a liquid crystal display device.
Another object of the invention is to provide a method of producing a long dichromatic polarizing element capable of obtaining the chips of the dichromatic polarizing elements as described above with a good yield.
According to the present invention, the dichromatic polarizing element and the production method thereof of the following constitutions are provided and the above-described objects of the invention are attained thereby.
1. A dichromatic polarizing element comprising a water-soluble organic dichromatic dye selected from the group consisting of a bisazo compound represented by following formula (1), the tautomer thereof, and the salts thereof, wherein the dichromatic dye comprises aggregates oriented to a definite direction such that the dye can polarize a light passing therethrough, 
wherein R represents an unsubstituted or substituted phenyl group or an unsubstituted or substituted heteroaryl group, and X represents an unsubstituted or substituted phenylene group.
2. The dichromatic polarizing element described in the above-described constitution 1, wherein the bisazo compound represented by the formula (1) is a bisazo compound represented by following formula (2), 
wherein R represents an unsubstituted or substituted phenyl group or an unsubstituted or substituted heteroaryl group, and A and B, which may be the same or different, each represents a group or an atom selected from the class consisting of a hydrogen atom, an alkyl group having from 1 to 4 carbon atoms, an alkoxy group having from 1 to 4 carbon atoms, and a halogen atom.
3. The dichromatic polarizing element described in the above-described constitution 2, wherein A and B are a same alkoxy group having from 1 to 4 carbon atoms.
4. The dichromatic polarizing element described in the above-described constitution 3, wherein A and B are a methoxy group.
5. The dichromatic polarizing element described in the above-described constitution 4, wherein R is a phenyl group substituted by one or more halogen atom.
6. The dichromatic polarizing element described in the above-described constitution 5, wherein R is a phenyl group substituted by one or more chlorine atom.
7. The dichromatic polarizing element described in the above-described constitution 2, wherein the bisazo compound represented by the above-described formula (2) is the bisazo compound selected from the group consisting of the compounds represented by following formula (2-1), (2-2), (2-3), (2-4) or (2-5): 
8. A method of producing the dichromatic polarizing element described in one of the above-described constitutions 1 to 7, which comprises orienting the aggregates of the dichromatic dye by a force capable of orienting the aggregates to a definite direction such that the dichromatic dye can polarize a light passing therethrough, and removing the force capable of orienting while leaving the aggregates as in the oriented state.
9. The production method described in the above-described constitution 8, wherein said dichromatic dye is the bisazo compound selected from the group consisting of the compounds represented by following formula (2-1), (2-2), (2-3), (2-4) or (2-5): 
10. The production method described in the above-described constitution 8, wherein the force capable of orienting is a shearing force.
11. A method of producing the dichromatic polarizing element described in one of the above-described constitutions 1 to 7, which comprises coating a solution of dispersing the aggregates of the dichromatic dye on a support by a die coating method, a blade coating method or a bar coating method.
12. The method of producing the dichromatic polarizing element described in the above-described constitution 11, which comprises coating a solution of dispersing the aggregates of the dichromatic dye on a support by a bar coating method.
13. The method of producing the dichromatic polarizing element described in the above-described constitution 12 wherein a bar used for the bar coating method is rotated to be disposed at a desired angle to the lengthwise direction of the support at coating the solution, whereby the aggregates of the dichromatic dye have an absorption axis of the desired angle.
14. The method of producing the dichromatic polarizing element described in the above-described constitution 11, wherein the concentration of the dichromatic dye in the solution is from 1 to 50%.